Diatoms are responsible for an estimated 40% of marine primary production and are therefore important players in global carbon cycling (Nelson et al. (1995) “Production and Dissolution of Biogenic Silica in the Oceans: Revised Global Estimates, Comparison with Regional Data and Relationship to Biogenic Sedimentation,” Global Biogeochem. Cycles 9(3): 359-372; Falkowski et al. (2004) “The evolution of Modern Eukaryotic Phytoplankton,” Science 305: 354-360). Though diatom growth in the oceans is thought to be controlled primarily by nitrogen and iron availability, recent studies support long standing hypotheses that cobalamin availability can impact marine phytoplankton growth and community composition (Boyd et al. (2007) “Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and Future Directions,” Science 315: 612-618; Moore et al. (2004) “Upper Ocean Ecosystem Dynamics and Iron Cycling in a Global Three-dimensional Model,” Global Biogeochem. Cycles 18: GB4028, doi:10.1029/2004GB002220; Panzeca et al. (2006) “B Vitamins as Regulators of Phytoplankton Dynamics,” Eos Trans. AGU, 87(52): 593-596; Bertrand et al. (2007) “Vitamin B12 and Iron Co-Limitation of Phytoplankton Growth in the Ross Sea,” Limnology and Oceanography 52(3)1079-1093; Gobler et al. (2007) “Effect of B-Vitamins and Inorganic Nutrients on Algal Bloom Dynamics in a Coastal Ecosystem,” Aquat. Microb. Ecol. 49: 181-194; Koch et al. (2011) “The Effect of Vitamin B12 on Phytoplankton Growth and Community Structure in the Gulf of Alaska,” Limnol. and Oceanog. 56: 1023-1034; Cowey C B (1956) “A Preliminary Investigation of the Variaton of Vitamin B12 in Oceanic and Coastal Waters,” J. Mar. Biol. Ass. UK, 35: 609-620; Droop (1957) “Vitamin B12 in Marine Ecology” Nature 180: 1041-1042; Menzel et al. (1962) “Occurrence of Vitamin B12 in the Sargasso Sea,” Limnol. Oceanogr. 7: 151-154). In the open ocean, cobalamin is present in exceedingly low concentrations and is depleted in irradiated surface waters, largely due to biological utilization (See, Menzel et al., supra).
Because no eukaryotic organism is known to produce cobalamin (Rodionov et al. (2003) “Comparative Genomics of the Vitamin B12 Metabolism and Regulation in Prokaryotes,” J. Biol. Chem. 278: 41148-41159), marine bacteria and archaea must therefore supply auxotrophic (vitamin-requiring) phytoplankton with the vitamin, either through direct interaction (Croft et al. (2005) “Algae Acquire Vitamin B12 Through a Symbiotic Relationship With Bacteria,” Nature 438: 90-93) or through production and release into the water column upon death and cell lysis (Droop M R (2007) “Vitamins, Phytoplankton and Bacteria: Symbiosis or Scavenging?” Journal of Plankton Res. 29: 107-113; Karl D M (2002) “Nutrient Dynamics in the Deep Blue Sea,” Trends in Microbiol. 10: 410-418). This chemical dependency is one of many that underlie interactions between marine microbial groups; assessing the role of these dependencies in oceanic processes is of considerable interest (Azam et al. (2007) “Microbial Structuring of Marine Ecosystems,” Nat. Rev. Microbiol. 5: 782-791). Cobalamin availability may play a significant role in the climatically important Southern Ocean where it appears to periodically colimit the growth of diatom-dominated phytoplankton communities (Bertrand et al. (2007), (supra)) and is likely in short supply relative to other marine environments (Bertrand et al. (2011) “Vitamin B12 Biosynthesis Gene Diversity in the Ross Sea: the Identification of a New Group of Putative Polar B12-Biosynthesizers,” Environmental Microbiology 13: 1285-1298).
The three available genome sequences of marine diatoms (P. tricornutum, T. pseudonana, and F. cylindrus) lack proteins homologous to known metazoan and bacterial cobalamin acquisition proteins (Koch et al. (2011) “The Effect of Vitamin B12 on Phytoplankton Growth and Community Structure in the Gulf of Alaska,” Limnol. and Oceanog. 56: 1023-1034). As a result, the mechanisms by which these phytoplankton acquire the vitamin from their environment remain unclear. Cobalamin requirements in eukaryotic algae, like diatoms, arise primarily from its use in the enzyme methionine synthase (Croft et al. (2005) (supra); Helliwell et al. (2011) “Insights into the Evolution of Vitamin B12 Auxotrophy from Sequenced Algal Genomes” Mol. Biol. Evol. 28(10):2921-33). Methionine synthase is responsible for generating methionine and tetrahydrofolate from homocysteine and 5-methyltetrahydrofolate, thus playing an essential role in cellular one carbon metabolism (Banerjee et al. (1990) “Cobalamin-dependent Methionine Synthase,” FASEB Journal 4: 1449-1459). Some eukaryotic algal genomes encode only one version of this enzyme, MetH, which uses methylcobalamin as an intermediate methyl group carrier (Goulding et al. (1997) “Cobalamin-dependent Methionine Synthase is a Modular Protein with Distinct Regions for Homocysteine, Methyltetrahydrofolate, Cobalamin and Adenosylmethionine,” Biochemistry 36: 8082-8091). These algae thus have an absolute cobalamin requirement. In contrast, other algal strains encode both MetH as well as MetE, an enzyme that accomplishes the same reaction as MetH but without cobalamin and with much lower efficiency (Gonzalez et al. (1992) “Comparison of Cobalamin-independent and Cobalamin-dependent Methionine Synthases from E. coli: Two Solutions to the Same chemical Problem,” Biochemistry 31: 6045-6056). Organisms with MetE and MetH thus have a flexible cobalamin demand and use cobalamin when available but do not absolutely require it. The maintenance of the much lower efficiency MetE enzyme in phytoplankton genomes presumably allows for ecological flexibility in environments with scarce or variable cobalamin availability (Helliwell et al. (2011) (supra)).
Once methionine is produced, it has several known fates within algal cells, including incorporation into proteins. Methionine also serves as the precursor to S-adenosyl methionine (AdoMet, SAM), an important methylating agent, propylamine donor, and radical source that participates in a wide range of cellular functions. Methionine can be used to produce another sulfur-containing metabolite dimethylsulfonium propionate (DMSP), which is only made by some diatoms, possibly as a cryoprotectant, osmolyte (Stefels J P (2000) “Physiological Aspects of the Production and Conversion of DMSP in Marine Algae and Higher Plants,” J. Sea Res. 43: 183-197) or antioxidant (Sunda et al. (2002) “An Antioxidant Function for DMSP and DMS in Marine Algae” Nature 418: 317-320), and is the precursor to the climatically important gas dimethylsulfide (DMS), and is the precursor to the climatically important gas dimethylsulfide (DMS) (Lovelock (1972) “Gala as Seen Through the Atmosphere,” Atmos. Environ. 6:579-580). In addition, impaired methionine synthase activity causes ‘methyl folate trapping’ whereby folate compounds can build up inside the cell in a form only usable by methionine synthase, thus preventing efficient folate recycling for use in its other essential functions such as nucleic acid biosynthesis. This phenomenon has been described in humans (Scott et al. (1981) “The Methyl Folate Trap: A Physiological Response in Man to Prevent Methyl Group Deficiency in Kwashiorkor (Methionine Deficiency) and an Explanation for Folic-Acid-Induced Exacerbation of Subacute Combined Degeneration in Pernicious Anaemia,” The Lancet 318: 337-340) and may also occur in algae (Croft et al. (2005) (supra)). The effects of cobalamin starvation on phytoplankton therefore potentially impact a wide range of cellular and ecological functions.